A conventional programmable gain amplifier (PGA) configuration is shown in FIG. 1. In such a PGA, an amplifier 10 receives an input signal at an input 11 and provides an output signal, determined by the gain of the amplifier, at its output 12. The gain of amplifier 10 is determined by the impedance in the feedback path to the feedback input 13 is determined by an impedance selector 14 located in the feedback path between the output 12 and the feedback 13. By varying the impedance in the feedback path using the impedance selector 14, the gain of the PGA may be varied. The impedance selector may be, for instance, a variable resistor, or, as is often used in integrated circuits, a switch and resistor network.
A conventional programmable gain amplifier using a switch and resistor network as the impedance selector 14 is shown in FIG. 2. In such a PGA, switches (S.sub.1 -S.sub.4) introduce errors in the signal path, due to both thermal voltage noise and voltage errors caused by amplifier input bias currents flowing through the switches. The input bias current flowing through the switches causes voltage variations due to a changing switch resistance with respect to the common-mode voltage. This non-linear resistance variation results in non-linear voltages biasing the input.
One way to decrease such errors in conventional complementary metal-oxide semiconductor (CMOS) PGA's is to increase the width of the CMOS switches, thus decreasing resistance. In an integrated circuit, however, an increase in the width of a switch results in an increased die area as well as additional capacitance. For low noise circuitry, the necessary safe area size for the switch can become a significant problem.
As an alternative, a second switch may be used to offset variation, in voltage caused by the first switch. This solution, however, increases both the noise caused by the switch and the die area of the integrated circuit.
Accordingly, it is an object of the present invention to provide a programmable gain amplifier that reduces errors associated with the switches.